Conventionally, there has been known a battery cell which includes an angular exterior can, electrode terminals mounted on an upper surface of the exterior can, and a waterproof sheet having insulating property which covers the exterior can (see JP 2011-175942 A). Edge portions of the waterproof sheet are made to overlap with each other on side surfaces of the exterior can, and the overlapped end portions of the waterproof sheet are thermally welded to each other thus forming the waterproof sheet into a bag having an opened upper portion. Although the waterproof sheet covers the side surfaces and a lower surface of the exterior can, an upper surface of the exterior can is not covered by the waterproof sheet so as to expose the electrode terminals and the like. Accordingly, an edge surface of the waterproof sheet is exposed upward, and a boundary between an outer surface of the exterior can and an edge surface of the waterproof sheet is also exposed upward.
With the configuration described above, in general, it is difficult to ensure sufficient adhesion between the exterior can and the waterproof sheet on a peripheral portion (upper end portion) of an opening of the waterproof sheet. Thus, a gap is liable to be formed at a boundary between the outer surface of the exterior can and the edge surface of the waterproof sheet. Since the boundary is exposed to the outside, there may be a case where a foreign substance (for example, water in the form of condensed dew) enters the gap from the outside. When the water which enters the gap expands due to the generation of heat in the battery cell, there is a possibility that the waterproof sheet is broken so that an insulating function of the waterproof sheet is impaired.